1. QOD
What is force?

3. Forces & the Laws of Motion
Physics I
Mr. Padilla

4. Aristotle (~400 B.C.) Divided motion into 2 types
Natural
Violent
Natural – Straight up or straight down.
Objects seek their “natural” resting place
Clouds – Up
Rocks – Down
Heavens – Perfect Circles around Earth
Violent – imposed motion
A result of forces that pushed or pulled. Had an external cause.
These ideas held for about 2000 years.

5. Copernicus ( )
Nicholas Copernicus was the first to reason that the Earth & planets moved around the sun.
Heavily persecuted for his ideas and worked in secret to hide them.
His work was published for the first time on the day he died.

6. Galileo Believed Copernicus and so was placed under house arrest
Force – any push or pull
Friction – any force that acts between materials as they move past each other
Only when friction is present is a force needed to keep an object in motion
Discredited Aristotle’s ideas of motion

7. Changes in Motion Forces can act through contact or at a distance
Contact forces – result from the physical contact between 2 objects.
Ex: Jordan pushes Kevin
Field forces – does not involve physical contact.
Ex: Gravity, electric fields
Field forces were developed to explain how an object can exert force on another without touching.

8. Newton Newton’s 1st Law of Motion
Every object continues in a state of rest, or of motion in a straight line at a constant speed, unless it is compelled to change that state by forces exerted upon it.
An object in motion stays in motion, an object at rest stays at rest.
AKA law of inertia

9. Inertia in Action
Inertia: the reluctance of any body to a change in the state of motion.
Book on a table
Air hockey table
Table cloth & dishes
Where would they go?
Ball thrown in outer space
Car without a driving force
Planets without the sun’s gravity

10. Mass vs. Weight The inertia of an object depends on its mass
How much material is in the object
The more mass = more inertia
Mass is a measure of the inertia of an object.
Mass is not weight.
Weight – force of gravity of an object
Weight can change in different environments: mass, and therefore inertia, remain constant.
Mass & Weight are proportional.

11. Weight
Weight can be calculated by taking an objects mass times gravity
W = mg
The SI unit for weight is
kg (x) m/s2 = kgm/s2 or Newtons (N)
A mass of 1kg has a weight of 9.8N, or 2.2lbs

12. Mass Example
Felicia, the ballet dancer, has a mass of 45.0 kg. a) What is Felicia’s weight on Earth? b) What is Felicia’s mass on Jupiter, where acceleration due to gravity is 25.0 m/s2? c) What is Felicia’s weight on Jupiter?

13. Net force Force is a vector quantity.
Has magnitude and direction.
Net force – When considering Newton’s 1st Law, you must think of net forces not just forces.
Net Force – combination of all forces acting on an object.
All individual forces can be added and treated as a single (net) force

14. FBD
A free body diagram can be drawn to represent the forces acting on an object.
A FBD shows vectors for forces acting on a single object

15. Equilibrium What forces act on a book at rest on a table?
At least 2 – weight and a support (normal) force
If the net force acting on an object is zero, it is in equilibrium.
No change in state of motion.
Normal force – always acts at a right angle to the surface.

16. QOD
Determine your weight, in Newtons, on the other planets in our solar system.
Mercury m/s2
Venus m/s2
Mars m/s2
Jupiter m/s2
Saturn m/s2
Uranus m/s2
Neptune m/s2

17. Acceleration So far: a = ΔV/t The cause of acceleration is force.
This is the definition of a
The cause of acceleration is force.
Applying a force (or net force) to an object causes it to accelerate
More force causes more acceleration (direct)
Mass resists the acceleration
more mass causes less acceleration (inversely)

18. 2nd Law
Acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object. Or
a ~ (net force)/mass a = F/m
Better yet… Fnet = ma

19. Examples
A force of 30.N is applied to a 12kg mass, how fast does it accelerate?
How much force is needed to accelerate a 700.kg vehicle at a rate of 4.0m/s2?
F = ma
30.N = 12kg(a)
a = 30N/12kg
a = 2.5 m/s2
F = (700.kg)(4.0m/s2)
F = 2800N

20. More Great Examples
Ben and Sam push on opposite sides of Chris. Ben pushes with 400.N of force, Sam with 200.N. What is the net force on Chris?
If Chris has a mass of 75kg, what will his acceleration be?
A constant force exerted on an object, will result in what kind of motion?
Fnet = ΣF acting on Chris
Fnet = 400.N – 200.N
Fnet = 200.N
F = ma
200N = (75kg)a
a = 200N/75kg
a = 2.7 m/s2
Constant force produces constant acceleration.

21. Free Fallin’
In a vacuum all things fall at the same rate due to gravity.
In reality, air resistance provides friction and causes some things to slow down.
All objects reach a point where the force of gravity pulling them down is equal to the force of air resistance pushing back on them.
This is called terminal velocity.
Humans = km/h in free fall
15-20 km/h with a parachute.

22. Forces
Forces are not a thing by themselves, but part of an interaction.
Forces always occur in pairs.
One force is called the action force, the other is called the reaction force.

23. Newton’s Third Law
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
To every action, there is an equal and opposite reaction.

25. Action/Reaction Pairs
When a hammer hits a nail, which is the action and which is the reaction force?
Identify the action/reaction forces when you walk.
Action: Object A exerts force on object B
Reaction: Object B exerts force on object A

26. Different Masses
Forces objects exert on each other are always equal in strength and in opposite directions.
Newton’s second law (F=ma) still applies.
If the masses are greatly different, then the object with the smaller mass will undergo greater acceleration.

27. Examples
A gun with a mass of 20kg shoots out a .04kg bullet. The gun kicks back at .2m/s2, at what rate does the bullet accelerate?
F = F
ma = ma
(20kg)(.2m/s2) = (.04kg)a
4N = (.04kg)a
100m/s2 = a
Calculate the kick of a 50kg cannon that shoots a 3kg cannonball out with an acceleration of 12m/s2.

28. Action - Reaction
A car accelerates along a road. Strictly speaking, what is the force that moves the car?
We know that Earth pulls on the moon. Does the moon also pull on the Earth? If so, which pull is stronger?
Can you identify the action and reaction forces of an object falling in the vacuum of outer space?

29. Cancellation Do action/reaction forces cancel each other out?
Create a system to determine net force on an object.
Ex: Boy on a skateboard, pulling against a girl standing on the ground.
Draw systems, identify action reaction forces.

30. Friction
Different surfaces have different amounts of friction between them (μ)
Friction is not restricted to solids , it also occurs in fluids
This can be felt even at slow speeds
Air resistance is friction acting on something moving through air.
Friction is a force that affects motion.
It acts when materials are in contact with one another and always opposes motion.
Friction is due to the irregularities in the two surfaces.
Constant speed = no net force so friction = driving force

31. Friction Two types of Friction
Static friction (Fs)
Restrictive force when object is not in motion
Fs = -Fa
Kinetic friction (Fk)
Restrictive force when object is in motion
Net external force acting on an object is equal to the difference between applied force and force of kinetic friction
F - Fk
Kinetic friction is less than static friction.

32. Friction The force of friction is proportional to the normal force.
Easier to push a chair across the floor at a constant speed than to push a heavy desk at the same speed.

33. Coefficient of Friction is a Ratio of Forces
Defined as the ratio between the normal force and the force of friction between two surfaces.
Coefficient of Kinetic Friction
μk = Fk Fn
Coefficient of Static Friction
μs = Fs,max

34. If the value of μ and the normal force on the object are known, then the magnitude of the force of friction can be calculated directly.
Ff = μFn

35. Coefficients of Friction
Material μs μk
steel on steel
Aluminum on steel
Rubber on dry concrete
Rubber on wet concrete
Wood on wood
Glass on glass
Waxed wood on wet snow
Waxed wood on dry snow
Metal on metal (lubricated)
Ice on ice
Teflon on teflon
Synovial joints in humans

36. Air Resistance is a form on friction
Whenever an object moves through a fluid medium (air or water), that fluid provides a resistance to the object’s motion.
Car at low speed = FR is roughly proportional to the car’s speed in the opposite direction
Car at higher speed = FR is roughly proportional to the square of the car’s speed in the opposite direction
When the magnitude of FR equals the magnitude of the force moving the car forward, the net force is zero and the car moves at a constant speed.

37. Examples
A 24 kg crate initially at rest on a horizontal floor requires 75 N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.
A student moves a box of books by attaching a rope to the box and pulling with a force of 90.0 N at an angle of 30.0o. The box of books has a mass of 20.0 kg, and the coefficient of kinetic friction between the bottom of the box and the sidewalk is Find the acceleration of the box.

38. Ch. 4 Mixed Review Answers
N to the right N down the incline 49. a N b N 51. a. 0 b N m 55. a m/s2 b c N d m/s
N N o 63. a. 20. m/s2 to the rt. b. F1 = 40. N F2 = 60. N F3 = 80. N c. m2 by m1 = 140 N m1 by m2 = -140 N m3 by m2 = 80. N m2 by m3 = -80. N 65. a. between the bottom block and the table. b. 0.47